A wavelength optical transmitter is an optical amplifier which amplifies light having a required wavelength band. Multiple wavelength optical transmitters are typically used in a system to transmit several laser signal lights over a single fiber. Each laser signal light represents a separate channel. It is critical that each laser signal light transmitted over a single fiber be on wavelength, otherwise they will interfere with each other. When two or more laser signal lights interfere with each other the light becomes a noise component and degrades the signal to noise ratio of the laser signal light. An optical wavelength filter is conventionally provided on the post-stage of the wavelength optical amplifier to prevent any laser signal lights from interfering with each other.
Another method used to stabilize the wavelength of the laser signal light is to improve the accuracy of the laser diode. However, improving the accuracy of the laser diode can be very expensive. To avoid these high costs, other methods for stabilizing the wavelength of a laser signal light are used. In one such method, a laser is first mounted to a thermoelectric cooler. A feedback control loop is then used to keep the thermister resistance of a thermoelectric cooler constant, thus holding the temperature of the laser transmitter constant. Since the laser transmitter temperature constant, the laser wavelength is stabilized. The above technique works well for optical transmitters requiring relatively constant output power, however, if the output power is varied, a simultaneous shift in the laser signal wavelength occurs causing interference with an adjoining channel.